It is highly desirable that paper toweling and personal care tissue-type products have a consumer perceived feel of softness, which is related to the product's bulk and density, and that the product be capable of readily absorbing liquids. These characteristics are related to the strength of the interfiber bonds within the paper web which occur as a result of the paper making process.
In the conventional felted wet press paper forming process, a liquid slurry of pulp, water and other chemicals is typically deposited on a Fourdrinier forming wire, transferred to a felt or fabric belt for drying and pressing, and thence transferred to a rotating Yankee drier cylinder which is heated to cause the paper to substantially dry on the cylinder surface. The moisture within the web as it is laid on the Yankee surface causes the web to adhere to the surface, and, in the production of tissue and toweling type non-woven products, the web is typically creped from the dryer surface with a creping blade. The creped web is then usually passed between calender rollers and rolled up prior to further converting operations. The action of the creping blade on the paper is known to cause a portion of the interfiber bonds within the paper to be broken up by the mechanical smashing action of the blade against the web as it is being driven into the blade. However, fairly strong interfiber bonds are formed between the wood pulp fibers during the drying of the moisture from the web. The strength of these bonds is such that, even after conventional creping, the web retains a perceived feeling of hardness, a fairly high density, and low bulk and water absorbency.
To reduce the strength of the interfiber bonds inevitably formed when wet pressing and drying the web from a slurry, various processes have been utilized. One such process is the passing of heated air through the wet fibrous web after it is formed on a wire and transferred to a pervious carrier--a so called through-air-dried process--so that the web is not compacted prior to being dried. The lack of compaction, such as would occur when the web is pressed while on the felt and against the drying cylinder when it is transferred thereto, reduces the opportunity for interfiber bonding to occur, and allows the finished product to have greater bulk than can be achieved in the conventional wet press process. Generally, the tensile strength of webs formed in the through-air-dried process is not adequate for a finished consumer product, and various types of bonders are typically introduced into the web in subsequent operations to achieve the desired strength while still retaining most of the bulk of the original product. Further reduction in the internal cohesion of the paper product may be obtained using various dry forming processes, such as air laying of substantially dry fibers onto a forming wire such that the resulting web has extremely low internal cohesion and very great bulk. Virtually all of the strength of such webs is obtained from the binders that are added to the web after forming. Because of the consumer perceived softness of these products, and their greater ability to absorb liquids than webs formed in conventional wet press processes, the products formed by the newer processes enjoy an advantage in consumer acceptance.
The conventional felted wet press process is significantly more energy efficient than processes such as through-air-drying and air laying of webs since it does not require the heating and moving of large quantities of air, as does the through-air-dried process, and does not require complete drying and fiberizing of the web as in the dry formed air laid processes. Excess moisture is mechanically pressed from the web and the final drying of the web is obtained chiefly on the heated Yankee drying cylinder which is maintained at the proper drying temperature with a relatively small expenditure of energy.
Some increase in the bulk of webs formed in the conventional wet press process has been obtained by utilizing chemical debonding agents which are added to the pulp furnish to inhibit the formation of the interfiber bonds. However, the use of chemical debonders in the furnish has not been observed to increase the bulk and absorbency of webs formed therefrom to the levels achieved in through-air-drying and air laying processes.